Vegetable oil based dielectric fluid and methods of using same

ABSTRACT

In one aspect, the present invention provides a dielectric fluid for use in electrical equipment comprising a vegetable oil or vegetable oil blend. In another aspect the invention provides devices for generating and distributing electrical energy that incorporate a dielectric fluid comprising a vegetable oil or vegetable oil blend. Methods of retrofilling electrical equipment with vegetable oil based dielectric fluids also are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 10/116,259, filed Apr. 4,2002, now U.S. Pat. No. 6,613,250, which is a continuation of Ser. No.09/288,877, filed Apr. 9, 1999, now U.S. Pat. No. 6,398,986, which is acontinuation-in-part of U.S. application Ser. No. 09/276,191 filed Mar.25, 1999, now U.S. Pat. No. 6,184,459 issued Feb. 6, 2001; which is adivisional of U.S. application Ser. No. 08/728,261 filed Oct. 8, 1996,now U.S. Pat. No. 6,037,537 issued Mar. 14, 2000; which is acontinuation of U.S. application Ser. No. 08/576,372 filed Dec. 21, 1995(abandoned).

FIELD OF THE INVENTION

In one aspect, the present invention relates to dielectric fluidcompositions, including insulating oils, for use in electricaldistribution and power equipment, including transformers, switchinggear, and electric cables. In another aspect the invention relates tovegetable oil-based insulating fluids and, more particularly, to the useof compositions comprising one or more vegetable oils. In yet anotheraspect, the present invention relates to the modification of electricaldistribution equipment in a manner that enhances their suitability forvegetable oil-containing dielectric fluid compositions.

BACKGROUND OF THE INVENTION

Dielectric (or insulating) fluids used in electrical distribution andpower equipment—including transformers, switching gear and electriccables—perform two important functions. These fluids act as anelectrical insulating medium, i.e., exhibit dielectric strength, andthey transport generated heat away from the equipment, i.e., act as acooling medium. When used in a transformer, for example, dielectricfluids transport heat from the windings and core of the transformer orconnected circuits to cooling surfaces. Apart from possessing dielectricstrength and cooling capacity, an ideal dielectric fluid for electricalequipment also exhibits little or no detrimental impact on theenvironment, is compatible with materials used to construct theequipment, and is relatively nonflammable.

For more than a century, mineral oils derived from crude petroleum wereused extensively as insulating and cooling liquids in electricalequipment. Though such oils possess a satisfactory dielectric strengthand are compatible with equipment materials, they are not considerednonflammable, and, because they are petroleum-based, they are consideredto carry with them an environmental cost. In the middle part of thiscentury, as safety standards became more demanding for many indoor andvault equipment installations, mineral oils were replaced to a largeextent by nonflammable liquids such as askarel (polychlorinatedbiphenyl, or “PCB”) fluids. Beginning in the 1930s, for example,PCBs—which generally are considered nonflammable—were used extensivelyto replace mineral oils in fire sensitive locations as insulating fluidsin electrical equipment.

PCBs eventually were recognized for their environmental hazards, and asa result the production and sale of PCBs as well as their use in newequipment was banned. For existing PCB-filled equipment, stringentregulations now require removal of PCB fluids at certain installationsand, for all other installations, place stringent restrictions on theuse of PCB-filled equipment. Spill reporting, clean-up, and disposal ofPCB-filled equipment also now require compliance with very strict EPAregulations.

Because of the disadvantages and shortcomings of PCB-based fluids andbecause of the increasing sensitivity to the potential adverseenvironmental impact of mineral oils and available alternatives, therehave been and continue to be numerous efforts undertaken to developrelatively inexpensive, environmentally safe, and nonflammabledielectric fluids. To date, these efforts have not been completelysuccessful.

There are a number of specific functional properties characteristic ofdielectric oils. An oil's dielectric breakdown, or dielectric strength,for example, provides an indication of its ability to resist electricalbreakdown and is measured as the minimum voltage required to causearcing between two electrodes at a specified gap submerged in the oil.The impulse dielectric breakdown voltage provides an indication of anoil's ability to resist electrical breakdown under transient voltagestresses such as lightning and power surges. The dissipation factor ofan oil is a measure of the dielectric losses in the oil; a lowdissipation factor indicates low dielectric loss and a low concentrationof soluble, polar contaminants. The gassing tendency of an oil measuresthe oil's tendency to evolve or absorb gas under conditions wherepartial discharge is present.

Because one function of a dielectric fluid is to carry and dissipateheat, factors that significantly affect the relative ability of thefluid to function as a dielectric coolant include viscosity, specificheat, thermal conductivity, and the coefficient of expansion. The valuesof these properties, particularly in the range of operating temperaturesfor the equipment at full rating, must be weighed in the selection ofsuitable dielectric fluids for specific applications.

In addition to the foregoing properties that affect heat transfer, adielectric fluid, to be useful in commercial applications, should have arelatively high dielectric strength, low dissipation factor, adielectric constant that is compatible with the solid dielectric, a lowgassing tendency, and it must be compatible with the electricalequipment materials to which it is exposed.

Current codes and standards require further that any dielectric fluidintended for use as a coolant not be classified as “Flammable,” butrather as a Class IIIB Combustible liquid. Specific safety requirements,however, vary with the application to which the electric equipmentcontaining the fluid is used. Such applications include, for example,indoor and rooftop installations, vault applications, and installationsadjacent to building structures. According to the degree of hazardattendant to these varied applications, one or more additionalsafeguards may be required. One recognized safeguard is the substitutionof conventional mineral oils with “less-flammable” and/or nonflammableliquids. Less-flammable liquids are considered to be those having anopen-cup fire point equal to or greater than 300° C.

Several dielectric fluids are known and used in electrical equipment.Due, however, to an increasing awareness and sensitivity towardenvironmental concerns, it has become increasingly desirable to providea dielectric fluid that: (1) poses minimal environmental hazards; (2)degrades quickly and easily so that spills do not contaminate the soilor the water table for any significant period of time; and (3) does notinterfere in any significant way with natural biodegradation processes.It also is becoming more desirable to replace non-renewable resourceswith renewable resources, particularly given the undesirability ofdependence on petroleum-derived products, and there generally isincreased demand by the industrial and retail markets for all-naturalproducts. This is due, at least in part, from the attention paid to thelong-term effects of materials and their degradation by-products.

In prior, related co-pending application Ser. No. 08/728,261—which isincorporated in its entirety by reference—we described a class ofinsulating dielectric fluids comprising vegetable oil materials. Thesecompositions, useful in electrical distribution and power equipment,utilize low maintenance vegetable oil-based dielectric coolants thatmeet or exceed applicable safety and performance standards and that arefree of substantial environmental hazards.

SUMMARY OF THE INVENTION

It is the general object of the present invention to provide electricalequipment utilizing an insulating liquid that is non-toxic,biodegradable, relatively inflammable, innocuous to the environment, andcomparatively inexpensive. In addition, the insulating oils typicallyconform to existing specifications and guides for dielectric fluids andmust exhibit performance characteristics that are generally comparableto presently used insulating oils.

In one aspect, the present invention provides a dielectric fluid for usein electrical equipment. The dielectric fluid comprises a vegetable oilor vegetable oil blend. In another aspect the invention provides devicesfor transforming, generating, and/or distributing electrical energy,including electrical transmission cables, switching gear andtransformers, that incorporate a dielectric fluid comprising a vegetableoil or vegetable oil blend. In yet another aspect, the inventionprovides methods of retrofilling electrical equipment with vegetableoil-based dielectric fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a cross-sectional view of a transformer tank housingincorporating a vegetable oil-based dielectric fluid and oxygenabsorption material housed in an oxygen permeable encasement.

FIG. 2 shows oxygen absorption material housed in an oxygen permeableencasement fastened to the tank cover of an electrical transformer.

FIGS. 3-4 provide cross-sectional views of a transformer tankincorporating a vegetable oil-based dielectric fluid and an oxygenabsorption material housed in an oxygen permeable encasement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As their most essential component, the dielectric fluids of the presentinvention comprise one or more vegetable oil compositions, many of whichare derived from plant matter. Vegetable oils typically comprise mixedglycerides formed from a polyol backbone, such as glycerin, in which theconstituent hydroxyl groups are esterified with an equal or nearly equalnumber of fatty acid molecules. Many useful vegetable oils aretriglycerides, i.e., are glycerides having three fatty acid moleculeschemically bonded to the glycerin backbone. Such triglycerides generallyare of the formula:

wherein R₁, R₂ and R₃ each, independently, is an alkyl or alkenyl groupthat may be straight-chained or branched, may be saturated orunsaturated, and may be unsubstituted or may be substituted with one ormore functional or non-functional moieties.

Differences in the functional properties of vegetable oils generally areattributable to the variation in their constituent fatty acid molecules.Several different fatty acids exist, including the following, all ofwhich may be present in the vegetable oils of the invention: myristic,palmitic, stearic, oleic, linoleic, linolenic, arachidic, eicosenoic,behenic, erucic, palmitiolic, docosadienoic, lignoseric, tetracossenoic,margaric, margaroleic, gadoleic, caprylic, capric, lauric,pentadecanoic, and heptadecanoic acids. These fatty acid molecules canalso vary in their degree of unsaturation.

Fatty acid molecules may be arranged on a polyol backbone in any numberof ways, and each polyol can have one, two or several differentconstituent fatty acid molecules. The three fatty acid molecules on atriglyceride molecule, for example, may be the same or may comprise twoor three different fatty acid molecules. While the compositions oftriglyceride compounds found in plant matter vary from species tospecies, and less so from strain to strain of a particular species,vegetable oil derived from a single strain of plant species generallywill have the same fatty acid composition.

Every naturally occurring triglyceride has a unique set of properties.For example, some triglycerides are more susceptible to oxidation thanare others. According to the present invention, it is preferred to useoils having fatty acid molecules that include a component having atleast one degree of unsaturation (i.e., at least one C═C double bond).This selection strikes a balance between the effects of oxidation with adesired reduction in the evolution of hydrogen gas. It has been foundthat oils containing mono-unsaturates oxidize less rapidly than dopolyunsaturated oils and are therefore somewhat preferred for use in thepresent invention. Specific, representative, vegetable oils suitable foruse in the present invention include the following: castor, coconut,corn, cottonseed, crambie, jojoba, lesquerella, linseed, olive, palm,rapeseed (canola), safflower, sunflower, soya, and veronia.

Useful vegetable oils preferably have an open-cup fire point well abovethe accepted minimum standard of 300° C. for both conventionaldielectric fluids and for “less-flammable liquids.” Several oils, forexample, typically have fire points of approximately 350° C. Accordingto the present invention, preferred oils have viscosities between about2 and about 15 cSt at 100° C. and less than about 110 cSt at 40° C. andhave heat capacities (specific heats) of greater than about 0.3 cal/g-°C. The vegetable oils of the invention also preferably have a dielectricstrength of greater than about 30 kV/100 mil gap, more preferablygreater than 35 kV/100 mil gap, and have a dissipation factor of lessthan about 0.05% at 25° C., more preferably less than about 0.03% at 25°C.

The vegetable oils of the invention may be used alone or may be blendedtogether with one or more other vegetable oils. In appropriatecircumstances, a vegetable oil or vegetable oil blend may also becombined with a minor amount of one or more synthetic oils, includingmineral oils. When a vegetable oil or vegetable oil blend is combinedwith one or more synthetic oils, the amount and/or character of thenon-vegetable oil component of the resulting blend should not interferewith the beneficial properties of the vegetable oil fluid. Thus, forexample, any significant amount of a chlorinated fluid (aromaticchlorinated compounds such as trichlorobenzene or polychlorinatedbiphenyls) will negate many of the positive environmental attributes ofthe vegetable oil component. Where such blends are employed, the blendshould contain less than 50 percent by weight of a petroleum-derivedmineral oil, preferably less than 30 percent by weight, more preferablyless than 20 percent by weight, and should contain less than 20 percentby weight of a chlorinated fluid, preferably less than 5 weight percent,and more preferably less than 1 percent by weight. It is also preferredthat the vegetable oil blend be “food grade,” i.e., that it not containany component that is considered toxic or otherwise biologicallyhazardous.

The vegetable oil and oil blends, where desired, may be pigmented orcolored with a suitable dye or pigment. Any known dye or pigment can beused for this purpose, and many are available commercially as foodadditives. The most useful dyes and pigments are those that are oilsoluble.

Because of its negative effect on dielectric performance, the presenceof water, a polar contaminant, in the vegetable oil-based fluid isundesirable. Water in the fluid tends to increase the rate of chemicalbreakdown of fatty acid esters in the vegetable oil in proportion to theamount of water available for such a reaction. The most obviousindicator of such reactions is a significant increase in the value ofthe neutralization number as measured by ASTM D974.

This problem can be compounded by the wide temperature range over whichelectrical distribution equipment must operate. It is known that thedielectric breakdown characteristics and other dielectric properties ofmineral oils are directly related to the percent saturation of waterpresent in the oil. The water saturation point of an oil is in turn afunction of temperature. As the saturation point is reached, dielectricstrength falls rapidly. The water saturation point for mineral oilstypically used as dielectric coolants is approximately 65 ppm at roomtemperature but over 500 ppm at normal operating temperatures (approx.100° C.). Electrical distribution equipment exposed to a wide variationin temperature can suffer a fluctuation in the degree of watersaturation in the dielectric fluid, and water that is dissolved or invapor/liquid equilibrium at high operating temperatures can precipitateor condense when the temperature of the oil decreases.

Currently accepted standards typically require the removal of moisturefrom conventional mineral oils to below about 35 ppm for use in newdistribution equipment. The moisture removal process uses eitherevaporation in a reduced pressure chamber or filtration, or acombination of both, to reach a level of between about 15 and 25 percentsaturation at room temperature (10-15 ppm) prior to filling of thedistribution equipment.

In contrast to mineral oils, vegetable oils generally have much highermoisture saturation points; typically over 500 ppm at room temperature.Therefore, acceptable moisture levels in vegetable oils used in newdistribution equipment can be much higher than those for conventionalmineral oils. Because the presence of water in vegetable oils can causethe additional breakdown of the constituent fatty acid esters, however,the moisture removal process used in the preparation of vegetableoil-based dielectric fluids should strive for moisture levels that reachbelow, as a percentage of saturation, those typically required formineral oils. A moisture level between about 5 and about 10 percent ofthe saturation level of water in the vegetable oil at room temperatureis preferred. The oils also are preferably processed by filtration orother suitable means to remove particulate and other contaminants. Thiscan be accomplished in a manner similar to the techniques for treatingand processing convention mineral oil-based dielectric materials.

Most useful vegetable oils are susceptible to polymerization uponexposure to free oxygen. Free oxygen activates unsaturated bonds in suchvegetable oils to begin an oxidative polymerization process. Suchpolymerization manifests itself in a marked increase in viscosity and acorresponding decrease in dielectric properties of the affected oil. Therate of this polymerization is, in part, a function of the temperatureof the oil at the time of exposure to free oxygen, and the by-productsproduced as a result of such polymerization are undesirable because theyhave chemical properties that are inferior to the virgin, orunpolymerized, oils. This degradation of vegetable oil by oxidativepolymerization is due to long-term exposure to free oxygen, and ittherefore can escape immediate detection.

The dielectric fluids of the invention optionally further comprise anoxidation reducing composition. Such compositions comprise one or morecompounds that absorb, or scavenge, oxygen that otherwise would dissolvein the vegetable oil composition and result in oxidative breakdown ofthe oil. When used, the oxygen absorbing compound is preferably encasedin a housing composed primarily of a polymeric material that issubstantially permeable to oxygen and substantially impermeable to waterand water vapor and that exhibits a high degree of mechanical strengththroughout the operating temperatures of the electrical equipment inwhich they are employed.

Useful oxidation reducing compounds are those that are capable ofreducing the concentration of free oxygen in the atmosphere surroundingthe dielectric fluid inside the sealed housing of electricaldistribution equipment and that in turn reduce the presence of dissolvedoxygen in the fluid itself. Such compounds can be referred to as oxygenscavenging compounds. Useful oxygen scavenging compounds include thosecommonly employed in the food packaging industry. Representative of theoxygen scavenging compounds useful in the practice of the inventioninclude the following: sodium sulfite; copper sulfate pentahydrate; acombination of carbon and activated iron powder; mixtures ofhydrosulfite, calcium hydroxide, sodium bicarbonate and activatedcarbon; a metal halide powder coated on the surface of a metal powder;and combinations of alkali compounds, such as calcium hydroxide, withsodium carbonate or sodium bicarbonate. Mixtures and combinations of oneor more of the above compositions are also considered useful.

Also useful as oxygen scavenging compounds are those compositionsprovided according to of U.S. Pat. No. 2,825,651, which is incorporatedby reference, including an oxygen remover composition comprising anintermixing of a sulfite salt and an accelerator such as hydrated coppersulfate, stannous chloride, or cobaltous oxide. Another useful class ofoxygen scavenging compounds are those compositions comprising a salt ofmanganese, iron, cobalt or nickel, an alkali compound, and a sulfite ordeliquescent compound, such as disclosed by U.S. Pat. No. 4,384,972,which also is incorporated by reference.

Preferred oxygen scavenging compounds include (or include as their basecomponent) at least one basic iron oxide, such as a ferrous iron oxide,or are made of mixtures of iron oxide materials. Useful ironoxide-containing compositions are available commercially, for example,under the “Ageless” trade name from the Mitsubishi Gas Chemical Companyof Duncan, S.C. and under the “Freshmax” trade name from MultisorbTechnologies, Inc. of Buffalo, N.Y. Also useful are oxygen absorbingagents comprising a mixture of ferrous salts and an oxidation modifierand/or a metallic sulfite or sulfate compound. Such compounds react withoxygen according to the following reaction mechanism:Fe→Fe⁺²+2e ⁻½O₂+H₂O+2e ⁻→2OH⁻Fe⁺²+2OH⁻→Fe(OH)₂Fe(OH)₂+½O₂+½H₂O→Fe(OH)₃

It should be noted that, in the reaction scheme outlined above, water isalso consumed, an advantageous benefit in the present application,because, as outlined previously, water is a polar contaminant that canitself adversely affect the dielectric properties of the vegetable oilswhen present in significant quantities.

The oxygen scavenging material is encased in a housing composedessentially of a polymeric material that exhibits a high permeability tooxygen and that also preferably exhibits a low permeability to water andwater vapor and that exhibits significant mechanical strength throughoutthe temperature range typically encountered in electrical distributionequipment. Specifically, useful polymeric materials are those that havean oxygen permeability of at least about 2,000 cc-mil/100 in²·24hrs·atm, preferably at least about 3,000 cc-mil/100 in²·24 hrs·atm, andmore preferably at least about 4,000 cc-mil/100 in²·24 hrs·atm.Permeability values for some representative polymers are provided, forexample, in G. Gruenwald, “Plastics: How Structure DeterminesProperties, p. 242 (Hanser, 1992). Useful polymeric materials alsopreferably have a tensile strength measured according to ASTM method D882 of at least about 3500 psi, more preferably at least about 4,000psi, and have a melting temperature higher than about 160° C.

Examples of suitable polymeric materials include polyolefins such ashigh density polyethylene, polypropylene, polybutylene, and copolymersthereof; polyphenylene oxide; polyethersulfone; nonwoven materials,including polyester felt; and cellulose pressboards. A particularlypreferred polymer material is polymethylpentene.

The encasement housing for the oxidation reducing composition may bemade from the polymeric material in any manner that permits theoxidation reducing composition to be in communication with thedielectric fluid headspace and allow for the direct exposure of anyoxygen in the environment with the oxidation reducing composition. Thehousing may be a simple pouch construction in which the oxidationreducing composition is encased within a film made of the polymericmaterial that is sealed to itself by ultrasonic welding, thermal sealingor other suitable sealing method, or the housing may be constructed ofmetal, hard plastic, or other suitable material and have a “window” offilm made of the oxygen permeable polymeric material through which theoxidation reducing composition communicates with the dielectric fluidheadspace.

The encasement housing may be placed inside the electrical distributionequipment in any configuration that allows for communication (throughthe polymeric material) between the oxidation reducing composition andthe dielectric fluid headspace. The housing thus may form an integralportion of the tank portion of the electrical equipment that holds thedielectric fluid. The housing may also be placed immediately inside andattached to the dielectric fluid tank portion of the electricalequipment and held there within the headspace of the tank.

The long term stability of the dielectric fluids of the invention may beimproved by utilizing any of the conventional methods known forimproving the stability or performance of dielectric fluids. Forexample, one or mare antioxidant or antimicrobial compounds may be addedto the dielectric fluid. Useful antioxidant compounds for this purposecan be dissolved directly in the dielectric fluid comprising thevegetable oil and include, for example, BHA (butylated hydroxyanisole),BHT (butylated hydroxytoluene), TBHQ (tertiary butylhydroxyquinone),THBP (tetrahydroxybutrophenone), ascorbyl palmitate (rosemary oil),propyl gallate, and alpha-, beta- or delta-tocopherol (vitamin E). It isgenerally also desirable to include in the dielectric fluid one or moreadditives to inhibit the growth of microorganisms. Any antimicrobialsubstance that is compatible with the dielectric fluid may be blendedinto the fluid. In some cases, compounds that are useful as antioxidantsalso may be used as antimicrobials. It is known, for example, thatphenolic antioxidants such as BHA also exhibit some activity againstbacteria, molds, viruses and protozoa, particularly when used with otherantimicrobial substances such as potassium sorbate, sorbic acid ormonoglycerides. Vitamin E, ascorbyl palmitate and other known compoundsalso are suitable for use as antimicrobial additives to the dielectricfluid.

The performance of dielectric fluids at low temperatures is important insome applications. Some vegetable oils do not, by themselves, have pourpoint values sufficiently low to be suitable for standard electricalpower distribution applications. Vegetable oils, unlike someconventional mineral oils, may also solidify or gel when cooled to atemperature just slightly above their pour point temperature for anextended period of time. A typical electrical power distributionapplication requires that a coolant have a pour point below about −20°C. The dielectric fluids of the invention, where insufficientthemselves, can be modified to ensure flowability at moderately lowtemperatures typically encountered during off-cycles (lower than about−20° C.). Suitable modification of the dielectric fluids include theaddition of a pour point depressant. Suitable pour point depressantsinclude polyvinyl acetate oligomers and polymers and/or acrylicoligomers and polymers.

Low temperature characteristics may also be improved by judiciousblending of oils. Certain oil blends, for example, have lower pourpoints than their individual constituent oils. For example, a blend of25 percent by weight soya oil (I) with 75 percent by weight rapeseed oil(II) has a pour point of −24° C., compared with −15° C. and −16° C. forthe constituent (I) and (II) oils respectively. Other vegetable oilblends that exhibit similarly advantageous reductions in pour pointsinclude: 25% soybean oil+75% oleate modified oil; 50% soybean oil+50%oleate modified oil; and 25% soybean oil+75% sunflower oil. It will beunderstood that this list of oil blends is not exhaustive and is offeredmerely to illustrate the nature of the invention.

The dielectric fluids of the invention preferably are introduced intothe electrical equipment in a manner that minimizes the exposure of thefluid to atmospheric oxygen, moisture, and other contaminants that couldadversely affect their performance. A preferred process includes dryingof the tank contents, evacuation and substitution of air with drynitrogen gas, filling under partial vacuum, and immediate sealing of thetank. If the electrical device requires a headspace between thedielectric fluid and tank cover, after filling and sealing of the tank,the gas in the headspace should be evacuated and substituted with aninert gas, such as dry nitrogen, under a stable pressure of betweenabout 2 and about 3 psig at 25° C.

It is preferable in any case to minimize or eliminate the presence ofoxygen in the headspace of the electrical equipment that contains avegetable oil-based dielectric fluid. There are several differentapproaches to the design of electrical equipment. One design thatgenerally is not suitable for the use of vegetable oil-based dielectricfluids is the conservator non-sealed type. A more common design type inANSI/IEEE standard electrical distribution and medium power equipmentemploys the use of a tank headspace to allow for the expansion andcontraction of the tank contents. Even if the headspace of the equipmentis purged of air and replaced with inert gases, it is possible over theoperating life for oxygen (air) to leak into the headspace from theopenings of the cover or accessories, the slow migration of air throughgaskets, and the operation of pressure relief devices. Ingress of oxygeninto the headspace will eventually contribute to the consumption of anyantioxidant additives in the fluid. It is also desirable therefore tominimize the presence of oxygen throughout the lifetime of theelectrical equipment through careful design and manufacture. One suchmethod for reducing the ingress of oxygen into the dielectric tank is toweld any components, covers, or access points that communicate with thetank headspace, as gaskets and other means for sealing such openings areall susceptible to leakage over time.

The dielectric fluids of the invention may be used in any applicationinto which conventional dielectric fluids are employed. Thus, thevegetable oil based fluids of the invention may be incorporated into alltypes of electrical equipment, including, but not limited to, reactors,switchgear, regulators, tap changer compartments, high voltage bushings,and oil-filled cables.

Cables that are used for the transmission and distribution ofelectricity generally incorporate a dielectric fluid, and are oftenreferred to simply as oil-filled cables. Oil-filled cables typicallycomprise at least one conductor around which there is provided a solid,stratified insulation formed by windings of insulation material tapesthat are impregnated with an insulating oil. These cables also generallyhave at least one longitudinal duct or canal that allows for themovement of the insulating oil along the length of the cable. Oil-filledcables are used both for underwater and land-based applications, andparticularly where submerged underwater, filled cables can be extremelysensitive to gasing tendency. Because they are most often pressurized,leakage from oil-filled cables can have a greater environmental impactfrom release of insulating fluid. The dielectric fluids of the presentinvention can be used to fill electrical cables. They can also be usedto retrofill cables that initially contain a non-vegetable oildielectric fluid.

Electrical transformers and switchgear typically are constructed byimmersing the core and windings and other electrical equipment in adielectric fluid and enclosing the immersed components in a sealedhousing or tank. The windings in larger equipment frequently are alsowrapped with a cellulose or paper material. The dielectric fluid of theinvention can be used to fill new electrical equipment in the mannerdescribed above. The fluids can also be used to retrofill existingelectrical equipment that incorporate other, less desirable dielectricfluids. Retrofilling existing equipment can be accomplished using anysuitable method known in the art, though because of the increasedsensitivity of vegetable oil fluids to moisture, it is important firstto dry components of the electrical equipment prior to the introductionof the vegetable oil based dielectric fluid. This is importantespecially with respect to the cellulose or paper wrapping, which canabsorb moisture over time. Because of the relatively high solubility ofwater in vegetable oils, a vegetable oil fluid can itself be used to dryout existing electrical equipment.

One method of retrofilling mineral oil containing transformers isdiscussed generally by Sundin in Retrofilling Mineral Oil TransformersWith Fire Resistant Fluids, Electricity Today, pp. 14-15 (May 1996),which is incorporated by reference. A useful method for retrofillingoil-filled electrical transmission cables is described in U.S. Pat. No.4,580,002, which is incorporated by reference. Other suitable methodswill be known by those skilled in the art.

The following descriptions and Figures are offered to provide anillustration of the invention and are given in reference to anelectrical transformer. It will be understood by those skilled in theart, however, that the compositions and methods encompassed by thepresent invention are equally suited for use in all types of electricalequipment, including those described above. These descriptions are to beunderstood as preferred and/or illustrative embodiments of the presentinvention and are not intended to limit the scope thereof.

Referring now to FIG. 1, a transformer tank 10 typically comprises atank body 12, a tank cover 14 bolted or welded to tank body 12 andsealed with gasket 16. Tank body 12 is sealed. Tank 10 houses thetransformer core and windings (not shown) or other electrical equipment,immersed in a dielectric fluid 18. The space between the surface of thefluid and the tank cover is the tank headspace 20. According to oneembodiment of the present invention, a polymer container 22 containingan oxidation reducing composition is mounted in the headspace of thetank, preferably on the inside of the tank cover as shown in FIG. 1. Asset forth above, container 22 is a pouch or bag encasement constructedof a oxygen permeable film.

A simple embodiment of the oxidation reducing composition is shown inFIG. 2 where a pre-packaged oxygen scavenging compound 8, such as isavailable commercially under the Ageless and Freshmax trade names, isencased in a “pouch” 22 constructed of a oxygen permeable polymer film,a polyester felt or a cellulose pressboard. The pouch is attached to thetank cover 14 by means of a simple clasp 6 or other suitable fasteningmeans. This embodiment finds particular utility in relatively smallelectrical equipment, such as in pole-mounted transformer assemblies.

According to another preferred embodiment shown in FIG. 3, the container22 is supported in a polyolefin housing 24 mounted adjacent to athreaded opening 26 in the tank cover. A threaded plug 28 seals thecontainer in the opening in the tank cover 14 and preferably includes atransparent view port 30. It will be understood that view port 30 canalternatively be incorporated into another part of the tank cover ortank wall.

When it is desired or necessary to replace the container containing theoxidation reducing composition, the threaded plug 28 can be removed, andthe container 22 removed from the polyolefin housing and replaced. Thelow gas permeability of housing 24 prevents significant gas exchangebetween the headspace 20 and the outside atmosphere during the shortperiod of time that the threaded plug is removed. This can beaccomplished even though the gas permeability of the container is not sohigh as to impede the operation of the oxidation reducing compositionover more extended periods of time.

Still in reference to FIG. 3, in addition to the oxidation reducingcomposition, it is preferred to provide a means for indicating thepresence of oxygen in the tank headspace. This indicator preferably isan oxygen sensitive compound 32 such as that marketed by Mitsubishi GasChemical Company under the trade name Ageless Eye. This compoundexhibits a pink-to-blue color change when the ambient oxygenconcentration exceeds 0.1%.

The oxygen indicator preferably is housed in the tank headspace wall insuch a manner that it can both chemically contact the gas in theheadspace and be visible for inspection from the outside of the tank.One way to accomplish this is to mount the oxygen indicator adjacent tothe view port 30 as shown.

In addition to the foregoing, the use of a vegetable oil-baseddielectric fluid in transformers can be facilitated through severalmodifications to the transformer tank. These include providing a sealed,accessible chamber such as described above in which the oxidationreducing composition can be replaced without increasing the exposure ofthe fluid in the tank to outside air. Other modifications reduce theleakage of the gas from within the tank, to thereby reduce the long-termexposure of the fluid to air.

Referring to FIG. 4, one such modification relates to the volume of thetank headspace 20. Current ANSI/IEEE C57 series standards, for example,require distribution transformer tanks to remain sealed over a top oiltemperature range of from −5° C. to 105° C. for pole-mounted andpad-mounted designs and over a 100° C. top oil range for substationtransformers. Outside this range the tank is typically vented to avoiddamage to the tank or related equipment. According to the presentinvention, the headspace volume is increased so that the temperaturerange over which the tank remains sealed increases correspondingly, thusreducing the probability of oxygen (air) leaking into the tank.Specifically, the present tank preferably includes a headspace volumesufficient to allow the tank to remain sealed from −20° C. to 115° C.

In addition, each tank includes an automatic pressure release device(PRD) 40 for venting the tank as described above. The PRD 40 can becalibrated to automatically vent headspace gas when the internalpressure exceeds acceptable levels, typically 9±1 psig, and toautomatically reseal when the pressure reduces to a desired level,typically to 6±1 psig. Because the PRD reseals at a positive pressure,the headspace will maintain a positive pressure even after venting bythe PRD. Maintaining a positive pressure in the headspace helps toprevent the ingress of air into the tank.

It is also preferable to replace conventional gaskets (not shown) withgaskets made from a material that is substantially gas impermeable. Itwill be understood that such gasket material must also be resistant todegradation by the dielectric fluid. Examples of suitable gasketmaterial include nitrile rubber with a high acrylonitrile content, andvarious fluoroelastomers, of which the compound sold under the tradename VITON from the E.I. du Pont Nemours & Company, is representative.Other suitable fluoroelastomers are available commercially from DyneonLLC of Oakdale, Minn. Materials with a relatively high gas permeability,such as silicone rubber and nitrile rubber having a low acrylonitrilecontent, are less suitable for gasket material. It will be understoodthat this list is illustrative only, and that other resilient, gasimpermeable materials could be used to form the gaskets for thetransformer tank. As mentioned above, another way to avoid the leakageassociated with the long-term use of gaskets is to weld the equipmenthousing shut and thereby eliminate completely gasketed seals.

Another method for reducing gas ingress is to reduce or eliminatealtogether the headspace and provide for thermal expansion by othermeans. The pressure/partial vacuum withstand would be based on a thermalrange of the average fluid temperature of about −20° C. through about115° C.

For units with sufficient headspace, vegetable oil-based dielectricfluids could also serve as excellent material in the recent developmentof High Temperature Transformers, which typically have a maximum top oilrated temperature rise over ambient of 115° C.

In addition to the foregoing, vegetable oil-based dielectric fluids inelectrical equipment in which paper insulation has been substituted bynon-cellulose insulating “paper” would have greater inherent stability.This is largely because cellulose materials liberate water as theydegrade thermally. Candidate materials include aramid insulatingmaterials, polyester materials, and polyamides.

While preferred embodiments of the invention have been shown anddescribed hereinabove, modifications thereof can be made by one skilledin the art without departing from the spirit and scope of the invention.

1. A transformer including a housing that contains a transformercore/coil assembly, comprising: a dielectric fluid surrounding saidcore-coil assembly, wherein the dielectric fluid consists of one or morevegetable oils and one or more antioxidant compounds, and wherein theone or more vegetable oils have a viscosity of between 2 and 15 cSt at100° C. and less than 110 cSt at 40° C., and wherein the dielectricfluid is environmentally safe.
 2. The transformer of claim 1, whereinthe one or more antioxidant compounds are selected from the groupconsisting of butylated hydroxyanisole (BHA), butylated hydroxytoluene(BHT), tertiary butylhydroxyquinone (TBHQ), tetrahydroxybutrophenone(THBP), ascorbyl palmitate, propyl gallate and alpha-, beta- ordelta-tocopherol.
 3. A transformer including a tank housing atransformer core/coil assembly, comprising: a dielectric fluidsurrounding said core-coil assembly, wherein the dielectric fluidconsists of one or more oleate modified vegetable oils and one or moreantioxidant compounds, and wherein the one or more vegetable oils have aviscosity of between 2 and 15 cSt at 100° C. and less than 110 cSt at40° C., and wherein the dielectric fluid is environmentally safe.
 4. Thetransformer of claim 3, wherein the one or more antioxidant compoundsare selected from the group consisting of butylated hydroxyanisole(BHA), butylated hydroxytoluene (BHT), tertiary butylhydroxyquinone(TBHQ), tetrahydroxybutrophenone (THBP), ascorbyl palmitate, propylgallate and alpha-,beta- or delta-tocopherol.
 5. A transformer includinga tank housing a transformer core/coil assembly, comprising: adielectric fluid surrounding said core-coil assembly, wherein thedielectric fluid consists of a base oil and additives that increase thefunctional properties of the base oil, the base oil consisting of one ormore vegetable oils having a viscosity of between 2 and 15 cSt at 100°C. and less than 110 cSt at 40° C., and the additives selected from thegroup consisting of one or more antioxidant compounds, a low temperatureadditive and an antimicrobial additive.
 6. The transformer of claim 5,wherein the one or more antioxidant compounds are selected from thegroup consisting of butylated hydroxyanisole (BHA), butylatedhydroxytoluene (BHT), tertiary butylhydroyquinone (TBHQ),tetrahydroxybutrophenone (THBP), ascorbyl palmitate, propyl gallate andalpha-, beta- or delta-tocopherol.
 7. A transformer including a housingthat contains a transformer core/coil assembly, comprising: a dielectricfluid surrounding said core-coil assembly, wherein the dielectric fluidconsists of one or more vegetable oils with a viscosity of between 2 and15 cSt at 100° C., and less than 110 cSt at 40° C., and one or moreantioxidant compounds; and wherein the dielectric fluid has: (a) aminimum dielectric breakdown of greater than or equal to 30 kV; (b) afire point of greater than 300° C.; and (c) a pour point between −15 and−25° C.
 8. The transformer of claim 7, wherein the one or moreantioxidant compounds are selected from the group consisting ofbutylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP), ascorbylpalmitate, propyl gallate and alpha-, beta- or delta-tocopherol.
 9. Thetransformer of claim 7, wherein the one or more vegetable oils areoleate modified vegetable oils.
 10. A method of using a transformerincluding a housing that contains a transformer core/coil assembly,comprising: employing in the transformer a dielectric fluid surroundingsaid core-coil assembly, wherein the dielectric fluid consists of a baseoil and additives that increase the functional properties of the baseoil, the base oil consisting of one or more vegetable oils having aviscosity of between 2 and 15 cSt at 100° C. and less than 110 cSt at40° C., and the additives selected from the group consisting of one ormore antioxidant compounds, a low temperature additive and anantimicrobial additive.
 11. The method of claim 10, wherein the one ormore antioxidant compounds are selected from the group consisting ofbutylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP), ascorbylpalmitate, propyl gallate and alpha-, beta- or delta-tocopherol.
 12. Amethod of using a transformer, comprising employing in the transformer adielectric fluid, the dielectric fluid consisting of one or morevegetable oils and one or more antioxidant compounds, wherein the one ormore vegetable oils have a viscosity of between 2 and 15 cSt at 100° C.and less than 110 cSt at 40° C., and wherein the dielectric fluid isenvironmentally sale.
 13. The method of claim 12, wherein the one ormore antioxidant compounds are selected from the group consisting ofbutylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP), ascorbylpalmitate, propyl gallate and alpha-, beta- or delta-tocopherol.
 14. Amethod of using a transformer, comprising employing in the transformer adielectric fluid, the dielectric fluid consisting of one or more oleatemodified vegetable oils and one or more antioxidant compounds, whereinthe one or more vegetable oils have a viscosity of between 2 and 15 cStat 100° C. and less than 110 cSt at 40° C., and wherein the dielectricfluid is environmentally safe.
 15. The method of claim 14, wherein theone or more antioxidant compounds are selected from the group consistingof butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT),tertiary butylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP),ascorbyl palmitate, propyl gallate and alpha-, beta- ordelta-tocopherol.
 16. A transformer including a housing that contains atransformer core/coil assembly, comprising: a dielectric fluidsurrounding said core-coil assembly, wherein the dielectric fluidconsists of one or more vegetable oils and one or more antioxidantcompounds, and wherein the one or more vegetable oils have a viscosityof between 2 and 15 cSt at 100° C. and less than 110 cSt at 40° C. 17.The transformer of claim 16, wherein the one or more antioxidantcompounds are selected from the group consisting of butylatedhydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP), ascorbylpalmitate, propyl gallate and alpha-, beta- or delta-tocopherol.
 18. Thetransformer of claim 16, wherein the one or more vegetable oils areoleate modified vegetable oils.
 19. A method of retrofilling atransformer, comprising removing an existing dielectric fluid from thetransformer and replacing the existing dielectric fluid with adielectric fluid consisting of one or more vegetable oils and one ormore antioxidant compounds, wherein the one or more vegetable oils havea viscosity of between 2 and 15 cSt at 100° C. and less than 110 cSt at40° C.
 20. The method of claim 19, wherein the one or more antioxidantcompounds are selected from the group consisting of butylatedhydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP), ascorbylpalmitate, propyl gallate and alpha-, beta- or delta-tocopherol.
 21. Themethod of claim 19, wherein the vegetable oils are oleate modifiedvegetable oils.
 22. A transformer including a housing that contains acore/coil assembly, comprising: a dielectric fluid surrounding saidcore/coil assembly, wherein the dielectric fluid consists of one or morevegetable oils, one or more antioxidant compounds and a low temperatureadditive, wherein the vegetable oils have a viscosity of between 2 and15 cSt at 100° C. and less than 100 cSt at 40° C., and wherein thedielectric fluid is environmentally safe.
 23. The transformer of claim22, wherein the vegetable oils are oleate modified vegetable oils.
 24. Atransformer including a housing that contains a transformer core/coilassembly, comprising: a dielectric fluid surrounding said core-coilassembly, wherein the dielectric fluid consists of one or more vegetableoils, one or more antioxidant compounds, and at least one of a lowtemperature additive and an antimicrobial additive, and wherein the oneor more vegetable oils have a viscosity of between 2 and 15 cSt at 100°C. and less than 110 cSt at 40° C., and wherein the dielectric fluid isenvironmentally safe.
 25. The transformer of claim 24, wherein the oneor more antioxidant compounds are selected from the group consisting ofbutylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tertiarybutylhydroxyquinone (TBHQ), tetrahydroxybutrophenone (THBP), ascorbylpalmitate, propyl gallate and alpha-, beta- or delta-tocopherol.
 26. Atransformer including a tank housing a transformer core/coil assembly,comprising: a dielectric fluid surrounding said core-coil assembly,wherein the dielectric fluid consists of one or more oleate modifiedvegetable oils, one or more antioxidant compounds, and at least one of alow temperature additive and an antimicrobial additive, and wherein theone or more vegetable oils have a viscosity of between 2 and 15 cSt at100° C. and less than 110 cSt at 40° C., and wherein the dielectricfluid is environmentally safe.
 27. The transformer of claim 26, whereinthe one or more antioxidant compounds are selected from the groupconsisting of butylated hydroxyanisole (BHA), butylated hydroxytoluene(BHT), tertiary butylhydroxyquinone (TBHQ), tetrahydroxybutrophenone(THBP), ascorbyl palmitate, propyl gallate and alpha-,beta- ordelta-tocopherol.
 28. A method of retrofilling a transformer, comprisingremoving an existing dielectric fluid from the transformer and replacingthe existing dielectric fluid with a dielectric fluid consisting of oneor more vegetable oils, one or more antioxidant compounds, and at leastone of a low temperature additive and an antimicrobial additive, andwherein the one or more vegetable oils have a viscosity of between 2 and15 cSt at 100° C. and less than 110 cSt at 40° C.
 29. A method ofretrofilling a transformer, comprising removing an existing dielectricfluid from the transformer and replacing the existing dielectric fluidwith a dielectric fluid consisting of one or more oleate modifiedvegetable oils, one or more antioxidant compounds, and at least one of alow temperature additive and an antimicrobial additive, and wherein theone or more vegetable oils have a viscosity of between 2 and 15 cSt at100° C. and less than 110 cSt at 40° C.